1. Technical Field of the Invention
This invention relates to metal containers and more particularly to an end closure for carbonated drink cans, and which may have an easy-open aperture.
2. Description of the Prior Art
Carbonated drinks are commonly contained in cylindrical metal containers having a top end closure of sheet material which is provided with a ring pull or tab for the easy opening of an aperture prescored in the end closure. In the manufacturing process, the cylindrical containers are filled and then fitted with their top end closures to enclose the drink, whereafter the containers may be subjected to heat in order to pasteurise the contents. This heating process causes a temporary increase in the pressure within the containers. Also if subsequently the container is stored in a warm environment or is severely shaken, its internal pressure will greatly increase. In order to withstand increased internal pressure, it is common for the top end closure of these containers to be formed with a circumferential valley or countersink adjacent its periphery. Although the containers may be designed to withstand an industry standard pressure of 100 p.s.i., it is possible for the countersink to invert at a localised point under internal pressure created within the container. There has hitherto been a tendency to minimise the thickness or gauge of the sheet material used to make the end closure, because the cost of the sheet material used in the end closure forms a substantial proportion of the overall manufacturing cost of the container. However in order that the gauge of the sheet material used for the end closure can be reduced, it has been necessary to take measures to prevent the countersink locally inverting under the internal pressure within the container. One proposal is given in U.S. Pat. No. 4,093,102 and relates to providing the countersink with a specific cross-sectional profile, such that the opposite walls of the countersink will move in response to internal pressure, the countersink thus adopting a modified profile having increased resistance to inverting. Another proposal is given in U.S. Pat. No. 4,832,223 and relates to coining the end closure around an annular region at the junction of the countersink with the centre panel of the end closure.